1.4 - Elements, Compounds & Mixtures

Elements, compounds and mixtures

Every substance is an element, a compound, or a mixture.

Elements

• An element is a substance made of only one type of atom.
• It can't be broken down into anything simpler by chemical means.
• There are about 100 different elements, all shown in the periodic table.
• The atoms in an element can exist as separate atoms (e.g. helium) or joined in pairs (e.g. oxygen gas, O₂).

Compounds

• A compound is a substance made of two or more different elements chemically combined in fixed proportions.
• The atoms are held together by chemical bonds, so separating a compound back into its elements requires a chemical reaction.
• A compound has a fixed chemical formula (e.g. H₂O always has two hydrogen atoms for every oxygen atom).
• A compound's properties are very different from the elements it's made from:
  • Iron is a magnetic metal and sulfur is a yellow powder.
  • Heated together, they react to form iron sulfide - a dull grey solid that isn't magnetic.

Mixtures

• A mixture contains two or more substances that are not chemically combined.
• There are no chemical bonds between the substances, so each keeps its own properties.
• The parts of a mixture can be separated by physical means (such as filtration or distillation) without a chemical reaction.
• Air and sea water are examples of mixtures.

Molecules

• A molecule is a particle made of two or more atoms joined together by covalent bonds.
• A molecule containing two atoms is a diatomic molecule.
• The atoms in a molecule can be the same element (e.g. oxygen, O₂) or different elements (e.g. water, H₂O).
• Single, separate atoms (like helium) are not molecules.

Molecule vs compound

• A particle is a molecule if it has two or more atoms covalently bonded together.
• A substance is a compound if it contains two or more different elements.
• For example:
  • Water (H₂O) is both a molecule and a compound.
  • Oxygen gas (O₂) is a molecule, but it is not a compound because it contains only one type of atom.

Classifying substances

From a chemical formula

• One element symbol → it's an element (e.g. He, O₂).
• Two or more different element symbols → it's a compound (e.g. H₂O, NaCl).
• A mixture contains multiple substances, so it can't be written as a single chemical formula.

From a particle diagram

• All particles alike:
  • Element if each particle contains only one type of atom.
  • Compound if each particle contains two or more different types of atom.
• More than one kind of particle → mixture.